Add MPS trace module implementation

This commit adds an implementation of the MPS trace module
based on `printf()`.

The enabling macro MBEDTLS_MPS_TRACE remains unset by default
because MPS tracing is very verbose and consumes unnecessary
space in the CI.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/library/mps/reader.c b/library/mps/reader.c
index 5c75c47..791b8bd 100644
--- a/library/mps/reader.c
+++ b/library/mps/reader.c
@@ -30,6 +30,10 @@
 #define inline __inline
 #endif
 
+#if defined(MBEDTLS_MPS_TRACE)
+static int trace_id = TRACE_BIT_READER;
+#endif /* MBEDTLS_MPS_TRACE */
+
 /*
  * GENERAL NOTE ON CODING STYLE
  *